The present invention relates to a color display device comprising a color display tube with a front portion having a truncated cone, means for generating a plurality of electron beams, a shielding of ferromagnetic material extending substantially parallel to the truncated cone, and a demagnetizing device having at least two demagnetizing coils extending along the outer circumference of the cone.
The invention also relates to a method for demagnetizing the shielding of a display device, so called degaussing.
A color display device of this type is well known in the art. The purpose of the shielding of ferromagnetic material is to reduce the effect of the earth""s magnetic field on the picture display. Without this shielding, the electron beams are affected by the earth""s magnetic field, causing errors in the landing of the beams.
When the display is switched on, the screen is demagnetized by means of an alternating current with decreasing amplitude, which current is led through the coils arranged on the wall of the display tube, creating a magnetic field. Due to this treatment, the magnetization of the shielding is adapted to the prevailing direction of the earth""s magnetic field.
A problem with this type of demagnetization (also called degaussing) is that part of the shielding is not demagnetized, or insufficiently demagnetized, because the applied field is not parallel to the shielding. In fact, the field resulting from the two coils approximately takes on the shape shown in FIG. 1, and is clearly perpendicular to large parts of the shielding.
To solve this problem, U.S. Pat. No. 5,559,401 teaches a color display device wherein two coils are energized at different times. Through this solution, the shielding is demagnetized in two possibly overlapping xe2x80x9cshotsxe2x80x9d, resulting in the different magnetic fields illustrated in FIGS. 2a-c. Although the performance is significantly improved with this technique, there are still areas of the shielding that are not satisfactorily demagnetized, especially the corners.
It is an object of the present invention to provide a display device of the kind described by way of introduction, having an improved landing performance. A further object of the invention is to generate a degaussing magnetic field, which is substantially parallel to a larger part of the shielding surface.
To this end, the device according to the invention is characterized in that the demagnetizing device is provided with means for energizing the coils in a phase-shifted manner, resulting in a rotating magnetic field. The rotating field can be arranged to substantially coincide with the shape of the shielding, resulting in a magnetic field, which is parallel to the shielding over a larger part of the shielding surface. This improves the demagnetizing procedure, leading to better landing performance of the electron beams.
The coils are preferably connected in parallel with a common power source. As the rotating field ensures a satisfactory demagnetization, there is no need for two separate power sources.
The coils can be identical, with at least one of the coils being connected in series with a capacitor and/or a inductor. The capacitor or inductor affects the reactance in one of the branches connected in parallel, resulting in a phase difference.
Alternatively, the coils can have different electrical properties, including different reactance. While requiring different types of demagnetizing coils, the need for extra circuit components, such as capacitors, is eliminated. The different electrical properties can easily be achieved by using coils with a different number of windings, or a different wire diameter.
According to another aspect of the invention, a method for demagnetizing a shielding in a color display device of the above kind is characterized by energizing said coils simultaneously and with a phase difference, thereby generating a rotating magnetic field.